CHEVY

My V6 chevy truck has a 4.3L marine motor in it. It will start up fine one day and run great, the next day it loses spark and will not start at all. I have replaced the distributor cap, rebuilt whole distributor, new plugs, wires everything. I had the distributor modular tested and it said it was fine. Any suggestions as to what else it could be?

Ok the only way to be shure is to 1. On the day that it wont start take the #1 plug out and rest it on the block have a good friend turn it over and make shure that there is no spark if no spark check the wire that goes to the coil the ignition wire that has to be the problem.